Author Topic: Augmenting thrust at launch  (Read 9431 times)

Offline sevenperforce

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Augmenting thrust at launch
« on: 04/04/2016 03:18 PM »
Staging works really well. Not only does it allow you to shed dry mass as you ascend, but it lets you use different engines at sea level than you do in space. This isn't just about expansion ratio. Because thrust/weight ratios and specific impulse tend to run inversely to each other, you can use a lower-efficiency but higher-thrust fuel for your launch engine cluster, while using a lower-thrust but higher-efficiency fuel for your upper stage. This allows you to dramatically reduce gravity drag and atmospheric drag, which more than makes up for the poorer fuel fraction of the higher-thrust fuel used in the first stage. The SSME delivered the vast majority of the Shuttle's dV but delivered something like a fifth of the total launch thrust, IIRC.

A reusable VTVL SSTO, then, has a major challenge. It must not only carry its entire dry mass to orbit and back while compensating for expansion ratios, but it must also find a way to deliver high takeoff thrust (to reduce drag losses) despite needing highly efficient fuel to keep the overall fuel fraction low.

Giving an SSTO two different engine clusters wouldn't make much sense. Nor does it make sense to simply give it a really really large engine with deep throttling, as the lower thrust-to-weight ratio of more efficient propellants would drive up dry mass fraction considerably. Even so, there are a few possible solutions.

Tripropellant engines. Design the same engine to burn different kinds of propellant, so that you can use a dense, high-thrust fuel like RP-1 at launch while using a lightweight, lower-thrust fuel like LH2 at altitude while still maintaining the same volumetric fuel flow rate. You could also use two different oxidizers, like high-test peroxide at launch and LOX at altitude.

There are some challenges; designing rocket engines for a single fuel combination is hard enough; using multiple fuels at multiple ratios is much harder, and there is a very real possibility that the decrease in efficiency required to accommodate multiple fuels will offset the benefits. You also have to add additional tankage and pumping systems for using three propellants rather than just two, and figure out the best way to fit these into your turbopump cycle. You need to decide whether to use three separate combustion chamber injectors or to premix/switch. You may still need to worry about altitude compensation.

Of course, there are advantages; if you pick something like HTP, you can use it to run your turbopumps as a monopropellant before cycling it into the combustion chamber, which is simpler than fuel-rich or oxygen-rich staged combustion. The same thing could be done using a monopropellant fuel, like hydrazine. You can offset the weight cost of additional tankage by using the same auxiliary fuel for your RCS system that you use to augment launch thrust.

Highly-variable mixture ratios. Engines have an optimal oxidizer:fuel ratio for the best specific impulse, but you could increase the amount of oxidizer (LOX being far denser than, say, LH2) and thus increase thrust at the expense of specific impulse. This requires your oxy turbopump to have a highly variable output, though, which is difficult to design. It is also less efficient than some other options and only moderately augments thrust, and depending on your flow characteristics you may have trouble with altitude compensation.

Auxiliary afterburning. Extend your expansion bell and add auxiliary injectors, and you can inject auxiliary propellant (fuel or oxidizer) to boost thrust. You'll want to vary your engine's mixture ratio to match the injection, of course; if you're injecting a dense oxidizer then burn fuel-rich; if you're injecting a dense fuel than burn oxygen-rich.

One major advantage here is that you can use a vacuum-optimized expansion bell at sea level because you will be injecting enough auxiliary propellant at launch to bring your exhaust pressure up above atmospheric pressure and thus prevent flow separation. Your need for augmented thrust will drop off as atmospheric pressure does, so you can reduce this gradually. This also allows your combustion chamber to be optimized for your high-efficiency primary fuel without needing to worry about mixing or injecting a third propellant as in a tripropellant design.

However, afterburning will occur at lower pressure and have lower efficiency than a tripropellant combustion chamber, increasing thrust-specific fuel consumption. The injectors also add an additional weight cost, and the exhaust bell must be quite large in order to have space for mixing, combustion, and expansion. So this will have a higher dry mass than a comparable tripropellant system. Other than that, the same factors apply with dual-use of auxiliary tankage and so forth.

Inert reaction mass injection. Rather than injecting auxiliary fuel, you can merely inject an inert working fluid into your exhaust stream, usually water. Although you are now carrying a propellant which does not contribute any chemical energy, you also don't have to worry about varying the mixture ratio of your actual engine (although the same is true of a hydrolox engine with HTP injection afterburning). You can inject quite a bit of water to raise thrust extremely high if you need to, and your dry mass is better because you only need space for mixing and expansion, rather than for combustion as with afterburning. Thrust and specific energy are now tied directly together; you can trade one for the other directly. Water injection will reduce the cooling requirements on your exhaust bell as well.

So those are the basic options. Are there any I missed? Which ones do you like, and which ones do you dislike? All factors being considered, which option (or combination of options) would be best for enabling a low-to-medium-payload VTVL reusable SSTO?

Offline TrevorMonty

Re: Augmenting thrust at launch
« Reply #1 on: 04/05/2016 06:47 PM »
A ULA slide shows a future LV with flyback engine pods attached to a disposal tank. A hybrid  SSTO could use same approach, not a true SSTO but simpler than a TSTO. 

Offline hkultala

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Re: Augmenting thrust at launch
« Reply #2 on: 04/05/2016 06:59 PM »
A ULA slide shows a future LV with flyback engine pods attached to a disposal tank. A hybrid  SSTO could use same approach, not a true SSTO but simpler than a TSTO.

There is NOTHING SSTO in that; if some engines are dropped it's parallel staging, which is still staging.
« Last Edit: 04/05/2016 07:01 PM by hkultala »

Offline sevenperforce

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Re: Augmenting thrust at launch
« Reply #3 on: 04/05/2016 07:31 PM »
A ULA slide shows a future LV with flyback engine pods attached to a disposal tank. A hybrid  SSTO could use same approach, not a true SSTO but simpler than a TSTO.

There is NOTHING SSTO in that; if some engines are dropped it's parallel staging, which is still staging.
Yeah, that's basically the other way to get full reuse; parallel stage. Something like this would work well:



But if you're going to go true SSTO then you need a way to augment launch thrust in your engines. I'm not sure whether I like water injection or afterburning better, myself...

Offline whitelancer64

Re: Augmenting thrust at launch
« Reply #4 on: 04/05/2016 07:34 PM »
Strap on some SRBs.
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Offline hkultala

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Re: Augmenting thrust at launch
« Reply #5 on: 04/05/2016 07:58 PM »
My favourite is tripropellant TAN;

Keep the main chamber burning hydrogen, and inject additional kerosine or methane and oxydizer into the nozzle.


Offline savuporo

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Re: Augmenting thrust at launch
« Reply #6 on: 04/05/2016 08:09 PM »
Aerospike ? Not exactly thrust augmentation, but an efficiency compensation
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Offline sevenperforce

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Re: Augmenting thrust at launch
« Reply #7 on: 04/05/2016 08:35 PM »
My favourite is tripropellant TAN;

Keep the main chamber burning hydrogen, and inject additional kerosine or methane and oxydizer into the nozzle.
That is essentially what I called "auxiliary afterburning" in the OP.

I wonder what kind of mass fraction a hydrolox SSTO using RP-1+HTP afterburning could achieve.

Aerospike ? Not exactly thrust augmentation, but an efficiency compensation
Still won't be enough to give hydrolox a high enough SL T/W ratio to avoid prodigious gravity drag. Especially since aerospikes typically have worse T/W ratios.

Offline hkultala

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Re: Augmenting thrust at launch
« Reply #8 on: 04/05/2016 08:42 PM »
My favourite is tripropellant TAN;

Keep the main chamber burning hydrogen, and inject additional kerosine or methane and oxydizer into the nozzle.
That is essentially what I called "auxiliary afterburning" in the OP.

I kinda considered it as combination of your "tripropellant" and "afterburning" options.


But what about TAN with solids?

Make a hydrogen staged cycle engine with a huge nozzle sized for good vacuum efficiency.
Then put some solid fuel inside the nozzle so that the hydrogen ignites it, it burns for about first minute or two giving some extra thrust(and preventing flow separation) and after that it is gone and the nozzle acts as ordinary vacuum nozzle. No need for additional pumps or piping for the TAN.

How is the burn speed controlled in solids? How easy or difficult would it be to make it burn at correct speed in this?
« Last Edit: 04/05/2016 08:49 PM by hkultala »

Offline Space Ghost 1962

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Re: Augmenting thrust at launch
« Reply #9 on: 04/05/2016 11:58 PM »
History of Liquid Propellant Rocket Engines
By George Paul Sutton

Quote from: George Paul Sutton
Page 626
In 1989, Energomash started the development of two tripropellant large booster engines. The RD-701 had two gimbaled TCs, and the RD-704 had a single TC. Both used a staged combustion engine cycle. The aim was to drive a single-stage-to-orbit launch vehicle using a combined first- and second-stage engine.
...
For the initial period of the flight (boost phase or ascent through atmosphere the engine burns both kerosene and LH2 with LOX at a high thrust level and a high chamber pressure. For the remainder of the flight (which is a sustalner phase) it burns only LH2 fuel with LOX at a much lower thrust and chamber pressure. The advantage of this dual fuel concept is a somewhat higher average fuel density, which allows a smaller total propellant tank volume, a slightly lower vehicle structure mass, a single TC for two propellant combinations, a lower drag. and a slightly improved vehicle performance.

Offline sevenperforce

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Re: Augmenting thrust at launch
« Reply #10 on: 04/06/2016 12:38 AM »

But what about TAN with solids?

Make a hydrogen staged cycle engine with a huge nozzle sized for good vacuum efficiency.
Then put some solid fuel inside the nozzle so that the hydrogen ignites it, it burns for about first minute or two giving some extra thrust(and preventing flow separation) and after that it is gone and the nozzle acts as ordinary vacuum nozzle. No need for additional pumps or piping for the TAN.

How is the burn speed controlled in solids? How easy or difficult would it be to make it burn at correct speed in this?
Good idea. The surface could be shaped as desired to provide whatever acceleration curve you wanted. Unfortunately, this would result in overexpansion if the same engine were used for propulsive landing. You could prevent chaotic flow separation by putting a double expansion nozzle in with inflection.

Unfortunately, specific impulse is really going to suffer because you are using unchoked flow.

A really good solution would be to use an open duct around the base and pack the bottom with solid fuel, so that by the time the solid fuel burns away you are moving fast enough to have ram effect air augmentation. That would make up for specific impulse losses by a long shot. The disadvantage, of course, is that you cannot adjust the amount of thrust to fit your mission, and "refueling" requires nozzle replacement.

The great thing about liquid propellant thrust augmentation is that it greatly increases the payload flexibility and, by extension, payload fraction. If you have a lighter payload you can use less augmentation fuel; if you have a heavier payload you can use more.

Offline ChrisWilson68

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Re: Augmenting thrust at launch
« Reply #11 on: 04/06/2016 01:45 AM »
Staging works really well.

[...]

A reusable VTVL SSTO, then, has a major challenge.

Well, the obvious conclusion is that we should keep using staging.

VTVL and SSTO should not be goals.  They should be among the possible choices for achieving the real goal: cheap, reliable access to space.

For expendable vehicles, single-stage-to-orbit is technically possible.  But it's far more expensive than using staging for any particular payload size.  That's why all expendable launch vehicles in use today use staging.  For the same reasons, unless there's some radical change in technology, every bit of evidence suggests that single-stage-to-orbit reusable vehicles will be far more expensive than using staged reusable vehicles.

Once we've nailed two-stage fully-reusable vehicles, then we can start trying to do SSTO reusable vehicles.  Until then, I hate to see effort and mindshare wasted on SSTO.  It's a misguided distraction.

We really, really, really need reusable orbital launch vehicles.  Lets take the easy path to that goal, not the hard path.

Offline sevenperforce

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Re: Augmenting thrust at launch
« Reply #12 on: 04/06/2016 03:28 PM »
Staging works really well.

[...]

A reusable VTVL SSTO, then, has a major challenge.

Well, the obvious conclusion is that we should keep using staging.

VTVL and SSTO should not be goals.  They should be among the possible choices for achieving the real goal: cheap, reliable access to space.
I agree. SSTO is not a magical curative; there is no reason why a fully-reusable TSTO launch vehicle could not have every bit as fast a turnaround time as an SSTO. Staging works, and we are moving toward reliable rapid reuse of the most expensive components.

But it's still a useful goal to pursue, although perhaps not with quite as much fervor as some do.

Launch costs can be roughly divided into three categories: vehicle costs, fuel costs, and ground support/infrastructure costs. Fuel cost is minimal compared to everything else, so that can be ignored. Reuse is aimed to reduce vehicle costs. Ground support and infrastructure, on the other hand, multiplies with the number of components you have in your launch vehicle. If you have two or three stages (including payload as a stage) then you don't merely have to check each one for flight readiness, but you have to check each of them in relation to each other. That's where an SSTO, if it were possible, would have a major advantage.

Plus, launch thrust augmentation has more applications than pure SSTO. An low-payload-SSTO fully-reusable booster with highly variable thrust would be able to serve double duty as a highly efficient first stage for larger payloads.

Offline savuporo

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Re: Augmenting thrust at launch
« Reply #13 on: 04/06/2016 04:00 PM »
Why derail a perfectly good tech thread into another reusability economics borefest ?

Here is another thrust augmentation idea:

http://www.gizmag.com/laser-propelled-ablation-space-rockets/34505/
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Offline sevenperforce

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Re: Augmenting thrust at launch
« Reply #14 on: 04/06/2016 06:29 PM »
Why derail a perfectly good tech thread into another reusability economics borefest ?

Here is another thrust augmentation idea:

http://www.gizmag.com/laser-propelled-ablation-space-rockets/34505/
I tend to dislike ground-based launch approaches but that is pretty cool. Might be useful if they could make it work, though I'm assuming clearance would be an issue.

If you must insist on staging then it would be interesting to go at it the opposite way...rather than making the first stage large and the second stage small, make the core stage capable of reusable SSTO with zero payload and then add a wrap-around parallel stage sized to the payload you need. It could provide crossfeed for the core stage and drop off to land propulsively with ease...

Offline RanulfC

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Re: Augmenting thrust at launch
« Reply #15 on: 04/07/2016 10:02 PM »
Why derail a perfectly good tech thread into another reusability economics borefest ?

Because economics are a big part of what drives the tech? :) More to the point the OP seemed to be directed at a single-point-design, (VTVL and SSTO) rather than general tech but sevenperforce cleared that up later.

And it is essential to keep in mind factors other than just payload and mass-fraction. (For example)

While thrust augmentation technically gives you a wider range of thrust options, at the same time in many cases it gives you gaps and low spots between with and without augmentation. The main reason strap-on SRBs are so wide spread is they can be tailored to fi the required thrust levels and then dumped when their job is done.

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Offline john smith 19

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Re: Augmenting thrust at launch
« Reply #16 on: 04/08/2016 08:14 AM »
A reusable VTVL SSTO, then, has a major challenge. It must not only carry its entire dry mass to orbit and back while compensating for expansion ratios, but it must also find a way to deliver high takeoff thrust (to reduce drag losses) despite needing highly efficient fuel to keep the overall fuel fraction low.
Yes it does. Why exactly do you want to do it this way?
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Tripropellant engines. Design the same engine to burn different kinds of propellant, so that you can use a dense, high-thrust fuel like RP-1 at launch while using a lightweight, lower-thrust fuel like LH2 at altitude while still maintaining the same volumetric fuel flow rate. You could also use two different oxidizers, like high-test peroxide at launch and LOX at altitude.
So far only the dual Oxidizer is anywhere close to going to a full size test bed engine.
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Highly-variable mixture ratios. Engines have an optimal oxidizer:fuel ratio for the best specific impulse, but you could increase the amount of oxidizer (LOX being far denser than, say, LH2) and thus increase thrust at the expense of specific impulse. This requires your oxy turbopump to have a highly variable output, though, which is difficult to design.
Not necessarily. The J-2 used this and they when with (IIRC) constant speed and flowrate LO2 turbopumps. The variable flow was done (like the RL10) by having a bypass valve on the LH2 outlet so a chunk of the LH2 flow would go back to the inlet.

Note that during the life of the Saturn V it's payload to the Moon went up 5%, half of that was done by varying the MR and that was only with 2 settings. Multiple steps should do even better.  The idea is to burn the heavier propellant (usually the oxidizer, but not always) first.
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It is also less efficient than some other options and only moderately augments thrust, and depending on your flow characteristics you may have trouble with altitude compensation.
True.
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Auxiliary afterburning. Extend your expansion bell and add auxiliary injectors, and you can inject auxiliary propellant (fuel or oxidizer) to boost thrust. You'll want to vary your engine's mixture ratio to match the injection, of course; if you're injecting a dense oxidizer then burn fuel-rich; if you're injecting a dense fuel than burn oxygen-rich.
Actually most nozzles are designed for maximum efficiency at altitude, so are substantially over expanded at SL. Running fuel rich is standard for most rocket engines (and no flown engine AFAIK has run the MR through the stochiometric range from fuel rich to rich poor. You just need to add the Oxidizer injectors.
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One major advantage here is that you can use a vacuum-optimized expansion bell at sea level because you will be injecting enough auxiliary propellant at launch to bring your exhaust pressure up above atmospheric pressure and thus prevent flow separation. Your need for augmented thrust will drop off as atmospheric pressure does, so you can reduce this gradually. This also allows your combustion chamber to be optimized for your high-efficiency primary fuel without needing to worry about mixing or injecting a third propellant as in a tripropellant design.

However, afterburning will occur at lower pressure and have lower efficiency than a tripropellant combustion chamber, increasing thrust-specific fuel consumption. The injectors also add an additional weight cost, and the exhaust bell must be quite large in order to have space for mixing, combustion, and expansion. So this will have a higher dry mass than a comparable tripropellant system. Other than that, the same factors apply with dual-use of auxiliary tankage and so forth.
Depends. In effect you're filling a vacuum space where most of the chamber pressure has already been lost in expansion. The injectors don't need that high a pressure. You seem to be looking to justify tripropellant as a design choice.
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Inert reaction mass injection. Rather than injecting auxiliary fuel, you can merely inject an inert working fluid into your exhaust stream, usually water. Although you are now carrying a propellant which does not contribute any chemical energy, you also don't have to worry about varying the mixture ratio of your actual engine (although the same is true of a hydrolox engine with HTP injection afterburning). You can inject quite a bit of water to raise thrust extremely high if you need to, and your dry mass is better because you only need space for mixing and expansion, rather than for combustion as with afterburning. Thrust and specific energy are now tied directly together; you can trade one for the other directly. Water injection will reduce the cooling requirements on your exhaust bell as well.
This has been done on some of the earlier Ariane engines, which until A5 were all storable hypergols.
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So those are the basic options. Are there any I missed? Which ones do you like, and which ones do you dislike? All factors being considered, which option (or combination of options) would be best for enabling a low-to-medium-payload VTVL reusable SSTO?
You missed the idea of ejectors around the rocket engines. Note this tactic improves when you have more perimeter between the nozzle and the airflow you're dragging in, so many small nozzles (driven by a single set of turbo machinery) should substantially improve this idea. However AFAIK the most this done with are the Russian 4 chamber, 1 pump set design.  My instinct is a lot of nozzles on a plate, with essentially a pressed metal sheet duct plate in from fed from closeable side inlets.

BTW the USAFRL developed a flight weight plug nozzle in the RL10 range driven by a dual expander (LH2 and LO2)  cycle in 1974. It met all test requirements but exploded due to water accumulation in welds due to poor selection of welding rods used in its mfg.
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Offline MikeAtkinson

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Re: Augmenting thrust at launch
« Reply #17 on: 04/08/2016 09:02 AM »
Launch costs can be roughly divided into three categories: vehicle costs, fuel costs, and ground support/infrastructure costs. Fuel cost is minimal compared to everything else, so that can be ignored. Reuse is aimed to reduce vehicle costs. Ground support and infrastructure, on the other hand, multiplies with the number of components you have in your launch vehicle.

Ground support and infrastructure also scale (exponentially?) with vehicle size. For a given size of payload a SSTO will be bigger both in dimensions and mass.

In my opinion the increased infrastructure and ground support for a SSTO due to its bigger size will outweigh the added complexity of a TSTO if both stages use the same fuel and similar (LS and Vac versions) engines.

This argument does not apply to Skylon as that achieves a reasonable payload mass fraction due to the SABRE engines in air breathing mode.

Offline sevenperforce

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Re: Augmenting thrust at launch
« Reply #18 on: 04/08/2016 03:35 PM »
Why derail a perfectly good tech thread into another reusability economics borefest ?

Because economics are a big part of what drives the tech? :) More to the point the OP seemed to be directed at a single-point-design, (VTVL and SSTO) rather than general tech but sevenperforce cleared that up later.
The example of VTVL SSTO was chosen not to restrict discussion, but more as a useful target. If you can come up with a good design for a reusable SSTO, it will work even better as a reusable TSTO/1.5STO, and have more flexibility.

SpaceX seems to have gotten "traditional" TSTO (big booster, small second stage) down to a science, but I'm interested in a reversed arrangement where you have a large SSTO-capable spacecraft with comparably smaller parallel-staged boosters to give it high payload capacity.

Tripropellant engines. Design the same engine to burn different kinds of propellant, so that you can use a dense, high-thrust fuel like RP-1 at launch while using a lightweight, lower-thrust fuel like LH2 at altitude while still maintaining the same volumetric fuel flow rate. You could also use two different oxidizers, like high-test peroxide at launch and LOX at altitude.
So far only the dual Oxidizer is anywhere close to going to a full size test bed engine.
What are the pros and cons of using dual oxidizers vs dual fuels? Obviously LOX is king if you use dual fuels, but if you're using dual oxidizers, what's the best fuel? LH2 is good; liquid methane might be better.

Auxiliary afterburning. Extend your expansion bell and add auxiliary injectors, and you can inject auxiliary propellant (fuel or oxidizer) to boost thrust. You'll want to vary your engine's mixture ratio to match the injection, of course; if you're injecting a dense oxidizer then burn fuel-rich; if you're injecting a dense fuel than burn oxygen-rich.
Actually most nozzles are designed for maximum efficiency at altitude, so are substantially over expanded at SL. Running fuel rich is standard for most rocket engines (and no flown engine AFAIK has run the MR through the stochiometric range from fuel rich to rich poor. You just need to add the Oxidizer injectors.
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One major advantage here is that you can use a vacuum-optimized expansion bell at sea level because you will be injecting enough auxiliary propellant at launch to bring your exhaust pressure up above atmospheric pressure and thus prevent flow separation. Your need for augmented thrust will drop off as atmospheric pressure does, so you can reduce this gradually. This also allows your combustion chamber to be optimized for your high-efficiency primary fuel without needing to worry about mixing or injecting a third propellant as in a tripropellant design.

However, afterburning will occur at lower pressure and have lower efficiency than a tripropellant combustion chamber, increasing thrust-specific fuel consumption. The injectors also add an additional weight cost, and the exhaust bell must be quite large in order to have space for mixing, combustion, and expansion. So this will have a higher dry mass than a comparable tripropellant system. Other than that, the same factors apply with dual-use of auxiliary tankage and so forth.
Depends. In effect you're filling a vacuum space where most of the chamber pressure has already been lost in expansion. The injectors don't need that high a pressure. You seem to be looking to justify tripropellant as a design choice.
Not trying to justify it, particularly; just suggesting different possibilities. You can definitely do dual propellant injection downstream.

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So those are the basic options. Are there any I missed? Which ones do you like, and which ones do you dislike? All factors being considered, which option (or combination of options) would be best for enabling a low-to-medium-payload VTVL reusable SSTO?
You missed the idea of ejectors around the rocket engines. Note this tactic improves when you have more perimeter between the nozzle and the airflow you're dragging in, so many small nozzles (driven by a single set of turbo machinery) should substantially improve this idea. However AFAIK the most this done with are the Russian 4 chamber, 1 pump set design.  My instinct is a lot of nozzles on a plate, with essentially a pressed metal sheet duct plate in from fed from closeable side inlets.
Air augmentation is great stuff; there's an active thread on that, or I would have included its mention here. Of course the possibility exists for combining air augmentation with something like water injection or auxiliary fuel injection, since ejectors don't offer a significant thrust boost at zero airspeed.

Offline john smith 19

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Re: Augmenting thrust at launch
« Reply #19 on: 04/08/2016 06:43 PM »
SpaceX seems to have gotten "traditional" TSTO (big booster, small second stage) down to a science, but I'm interested in a reversed arrangement where you have a large SSTO-capable spacecraft with comparably smaller parallel-staged boosters to give it high payload capacity.
If it stages it's not SSTO. It doesn't matter what is staged.

You seem to be looking at something more like a Biamese or a Triamese design, for which a thread was run
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What are the pros and cons of using dual oxidizers vs dual fuels? Obviously LOX is king if you use dual fuels, but if you're using dual oxidizers, what's the best fuel? LH2 is good; liquid methane might be better.
It depends what you're doing with it. In SABRE's case it's deeply pre cooling of the airflow to give near constant entry conditions at the compressor front face. Delivering that function with a different fuel depends on what their thermal properties are.  If you're not air breathing that changes the choice. 
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One major advantage here is that you can use a vacuum-optimized expansion bell at sea level because you will be injecting enough auxiliary propellant at launch to bring your exhaust pressure up above atmospheric pressure and thus prevent flow separation. Your need for augmented thrust will drop off as atmospheric pressure does, so you can reduce this gradually. This also allows your combustion chamber to be optimized for your high-efficiency primary fuel without needing to worry about mixing or injecting a third propellant as in a tripropellant design.
Yes, exactly.
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However, afterburning will occur at lower pressure and have lower efficiency than a tripropellant combustion chamber, increasing thrust-specific fuel consumption. The injectors also add an additional weight cost, and the exhaust bell must be quite large in order to have space for mixing, combustion, and expansion. So this will have a higher dry mass than a comparable tripropellant system. Other than that, the same factors apply with dual-use of auxiliary tankage and so forth.
Efficiency is a very loaded term.
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